The digital transmission system offers significant advantages over an analog system. In the digital system, the message is coded into a signal which can be regenerated rather than amplified at each repeater location. Normally, the digital system requires a greater bandwidth, but the digital system can operate with a lower signal to noise ratio. In addition, the cost of a digital terminal is less than that of an analog terminal.
The typical digital transmission system employs an encoder at a transmission location, which encoder functions to code a desired signal, which may be an analog signal (audio, speech and so on) into a digital signal which is then transmitted over the communications link or channel. The receiver consists of a decoder. The function of the decoder is to convert the signal back into its original form or the form the signal possesses prior to encoding. In this manner, the decoded signal can further be processed to provide the original analog signal or the signal information content. As such, the digital systems employ a series of pulses to carry the information over the channel. There are numerous ways of modifying the characteristics of a train of pulses to convey information. Such systems which are in widespread use are referred to as PCM or pulse code modulation systems. As one can ascertain, there are numerous techniques of modulating pulse trains to convey information which are encompassed within PCM systems.
A particular useful form of a PCM signal type is referred to as AMI or Alternate Mark Inversion. In this coding system, a transition occurs at the beginning of every binary bit or every symbol in a train of pulses. One binary value (as a binary one) is manifested by a second transition one-half period later, while a second value (binary zero) has no second transition. The AMI code provides a balanced representation of a PCM bit stream and tends to minimize the average DC (direct current) level on a transmission line, as compared to other codes. Examples of AMI systems and circuitry for encoding and decoding such signals is had by reference to U.S. Pat. No. 4,071,692 entitled DATA TRANSMISSION SYSTEMS issued on Jan. 21, 1978 to Donald A. Weir et al and assigned to the International Standard Electric Corporation.
In any event, as indicated above, the digital transmission system normally requires regeneration and decoding. The decoding process is typically and conventionally associated with a synchronous decoding scheme. In this manner, the received digital signal is processed to formulate a signal which is synchronized to the transmitted signal and therefore is "locked" or synchronized to the encoded signal or clock. In obtaining this signal, the prior art employed a retiming circuit such as a phase locked loop (PLL) or an injection locked oscillator (ILO). These retiming circuits, the PLL and the ILO have a long transient response and generally poor reliability. Such characteristics provide problems in reliable decoding and reliable system operation and obviously should be avoided. These problems are coupled with the additional circuitry required and hence, additional costs necessary to implement retiming functions.
It is therefore an object of this invention to provide an improved decoder particularly adapted to decode AMI signals, which decoder is extremely reliable and simple to implement and which decoder does not require retiming circuitry.